The use of polymer films as coatings is quite common. These films, frequently formed of polyvinyl alcohol and/or polyvinyl acetate, usually have a low degree of hardness which reduces their durability and scratch resistance. Besides, many of these films have poor resistance to moisture and solvents. Various means have been employed to overcome these deficiencies. These means usually include the treatment of the films with hardening solutions which contain substances which react with the film to give it the desired hardness. Such substances which include polyvalent inorganic metallic salts, organo-metallic compounds, aliphatic mono-and di-aldehydes, and the like, have long been used effectively to post harden photographic gelatin films, poly (vinyl alcohol) screen printing stencils, and lithographic printing plates. Examples of such hardening compositions and their use are disclosed in U.S. Pat. Nos. 2,407,061 and 2,720,468.
While these compositions have been of some use, such use has been limited because of certain deficiencies, such as a short shelf life, excessive toxicity and inadequate hardening effect. In particular, in the prior art compositions, the hardening aldehyde agents or precursors thereof have to be mixed with the strongly acidic aqueous or non-aqueous system just prior to use since the stability of these acidic solutions of the aldehydes or aldehyde precursors is measured only in hours. This requires the provision of a two-part system, the parts being mixed just prior to use.
U.S. Pat. No. 2,785,948 discloses compositions containing acetals of mono- or di-adelhydes, which compositions are useful in preventing shrinkage of cellulosic textile materials. These compositions contain large amounts of water, in the range from 46 to 95%, and are unsuitable for the purposes and practice of this invention.
It is, accordingly, an object of this invention to provide a single phase (i.e. a 1-part) composition which produces a good hardening effect when applied to organic polymer films.
It is another object of this invention to provide hardener compositions which are substantially non-toxic in normal use.
It is a further object of this invention to provide hardener compositions having a substantially unlimited shelf life.
Other objects will appear from the description which follows.